1. Field of the Invention
This invention relates to devices and methods for moving an implant in a body, and more particularly to such devices and methods that apply pushing forces with a flexible attachment and that magnetically steer the implant in the body with high accuracy.
2. Description of Related Art
There is a large body of conventional (nonmagnetic) stereotactic prior art, in which a frame (e.g., a so-called "BRW Frame") is attached to the skull to provide a navigation framework. Such a frame has arcs to determine an angle of an "insertion guide" which is usually a straight tube through which some medical therapeutic agent is passed, such as a biopsy tool. These methods have been confined to straightline approaches to a target.
There is also a smaller body of prior art in which a handheld permanent magnet or an electromagnet is used to move a metallic implant.
Previous implants for delivering medication or therapy to body tissues, and particularly brain tissue, have generally relied upon the navigation of tethered implants within vessels, or navigation of tethered or untethered implants moved intraparenchymally (in general brain tissue) by magnetic force.
Navigation of untethered implants, in the past, has generally comprised finding ways to apply magnetic force optimally, including both magnitude and direction, for a given step of "free" motion. However, difficulty in finding a set of currents to accomplish a move step is encountered because of the complexity of calculating the magnetic forces resulting from multiple coils.
It is well-known that two like coils on a common axis, having like currents, provide a highly uniform magnetic field on their axis at the midpoint between them. In addition, it is known that the field is approximately uniform for an appreciable region around the midpoint, and relatively strong, as compared with any other two-coil arrangement having the same coil currents. This arrangement of coils and currents meets the need for an accurate, strong guiding torque applied to a magnetic implant near the midpoint between the coils. Because the field is quite uniform near the midpoint, undesired magnetic forces on the implant are negligible. However, this arrangement is less suitable for a moving implant when the implant is some distance from the midpoint between the coils or not on the axis, or when the implant axis is not along the coil axis. In these important cases, this simple coil arrangement cannot provide accurate directional guidance. Furthermore, simple vector combinations of three such coil pair arrangements cannot provide accurate guidance in an arbitrary direction, except at one spot at the center of the arrangement.
The Magnetic Stereotaxis System (MSS) originated from the hopes that a less-invasive methodology could be developed which would allow neurosurgeons to operate in previously inaccessible regions of the brain. By introducing a small permanent magnetic implant into the brain through a small "burr hole" drilled through the skull prior to the operation, large superconducting coils could be used in conjunction with a pushing mechanism to magnetically guide the implant and overlaying catheter through the brain's parenchyma, all the while avoiding the important structures of the brain. The operational methodology of the MSS was, and continues to be, expected to be less destructive to the tissues of the brain than the shunts, straight tubes, and other devices associated with conventional techniques in neurosurgery.
The first MSS was conceptually developed in 1984 as the Video Tumor Fighter (VTF), and is shown in U.S. Pat. No. 4,869,247 issued Sep. 26, 1989. This system specifically focused on the eradication of deep-seated brain tumors via hyperthermia-based treatment. It was envisioned that the magnetic coils of the VTF would guide a small (.about.3 mm diameter) magnetic thermosphere through the brain into a tumor. Rastering the implant throughout the volume of the growth, the tumor cells could be destroyed by inductively heating the implant with radio-frequency radiation.
Further studies revealed that the reality of a magnetomotive based system used to direct a small implant promised numerous applications other than the hyperthermia-based treatment of brain tumors by induction. These included: biopsy, pallidotomy, delivery of precision radiation therapy, magnetically placed implants that deliver chemotherapy to otherwise inaccessible tumor locations, and (by attaching a semi-permeable catheter to the implant) the delivery of chemicals to specific sites in the brain without the need for penetrating the blood-brain barrier which has complicated contemporary systemic chemical delivery. This means of chemical delivery seemed particularly hopeful in the treatment of Parkinson's disease, where the catheter could be used to deliver dopamine to the affected regions of the brain with minimal indiscriminate distribution of the neurotransmitter to the surrounding tissue, thereby lessening attendant side effects. It was in the light of these possible broadened applications of the VTF that the system became known as the MSS.
Referring now to FIG. 1A and FIG. 1B, the most recent MSS apparatus 10 included six superconducting coils (not visible in FIG. 1A and FIG. 1B) located in a rectangular box or helmet 12. With the z-axis defined in the direction of the axial component of the head, the x- and y-coil axes are rotated 45.degree. from the sagittal plane 14 of the head, which would be positioned in opening 16. The x- and y-coil axes are symmetrically located such that the horizontal extensions 22 of the MSS apparatus 10 away from the patient's body is minimized. Because the lower edge of the treatable part of the brain is typically located 10 cm above the shoulder line for an average adult, the z-coils (located on the body-axis of the supine patient) were compressed to allow for a maximum extension of the head into helmet 12.
The vision component of the MSS consists of a superposition of pre-operative MRI images referenced by biplanar fluoroscopy cameras 20 linked to a real-time host system (not shown in FIG. 1A and FIG. 1B). Both cameras 20 are calibrated to the MSS six-coil helmet design. X-ray generators for cameras 20 are located inside magnetic shields 22. Using x-ray visible fiducial markers located on the skull of the conscious patient, the coordination of the implant's position inside the cranial volume to the helmet's reference system (and hence the corresponding preoperative MRI scan) is done through a series of coordinate transformations executed by a host system and displayed for the surgeon on a workstation.
The central problem to the inductively-based guidance of a magnetic implant pertains to the inverse problem of electromagnetism as influenced by Earnshaw's theorem. The conventional problem of electromagnetism centers on the evaluation of the gradient and magnetic field given established magnetomotive sources. For the MSS, however, the situation is reversed in that the magnetic field and its gradient are specified at a point in space while the strengths of the six actuators are to be determined. Control of the motion and position of an untethered implant would be difficult in the MSS, given the fundamental instability of a non-diamagnetic moment in a static or quasi-static magnetic field as related to Earnshaw's theorem for static/quasi-static magnetic fields, if it were not for the resistive nature of the parenchyma. In early tests, small cylindrical (up to 5 mm in length and 5 mm in diameter) permanently magnetized NdBFe objects were used. The relatively strong moment of these objects (0.016 A-m.sup.2 to more than 0.04 A-m.sup.2) facilitated the creation of the necessary aligning torque without the requirement of a strong magnetizing field, resulting in lower current values.
The permanent magnetization of the implant requires a predetermined magnetic field in order to ensure that the implant is oriented in the desired direction. While it is possible to generate a magnetic force to displace the implant, it was found that the requirement of specific force and field alignment could result in unobtainable currents (as high as thousands of amperes). It was also found that even for viable solutions, the equilibrium state was sometimes unstable to such an extent that the implant tended to be difficult to control.